A micromechanical yaw-rate sensor is frequently used to ascertain information about a rotation behavior of a rotatable body.
FIGS. 1A and 1B show schematic illustrations of a typical micromechanical yaw-rate sensor having a sensitive axis to explain its functionality. The typical yaw-rate sensor is described in German Patent Application No. DE 195 23 895 A1.
Illustrated yaw-rate sensor 10 has a disc-shaped oscillating mass 12, which is connected via multiple springs 14 to a hub 16. Of the total of four springs 14, however, only two are shown in FIG. 1A. Springs 14 are situated in a circular central opening of oscillating mass 12, into which hub 16 projects. The end of hub 16 oriented opposite to oscillating mass 12 is fixedly connected to a substrate 18. Hub 16 and substrate 18 are subunits of a holding device, in which oscillating mass 12 is oscillatably situated. Further components of the holding device are not shown in FIGS. 1A and 1B for a better overview.
Drive electrodes 20, formed as comb electrodes, are formed on oscillating mass 12, only one of which is shown in FIG. 1A, however. Two drive electrodes 22, which work together, are fixedly connected to main substrate 18 for each drive electrode 20 of oscillating mass 12. A voltage U may be applied between drive electrode 20 of oscillating mass 12 and an adjacent drive electrode 22 of main substrate 18 via a control unit of yaw-rate sensor 10. The application of voltage U between drive electrodes 20 and 22 causes a rotational movement of oscillating mass 12 around an oscillation axis 24 in relation to substrate 18, oscillation axis 24 running perpendicularly to the surface of disc-shaped oscillating mass 12.
Sensor electrodes 26, which have the shape of electrode combs, are also situated on oscillating mass 12. Further sensor electrodes 28 are fixedly connected to substrate 18 adjacent to sensor electrodes 26 of oscillating mass 12. Each sensor electrode 26 of oscillating mass 12 and sensor electrode 28 of substrate 18, which works together therewith, are coupled to a power circuit having a capacitor 30. A rotational movement of oscillating mass 12 around oscillation axis 24 causes a change in capacitance C1 of capacitor 30. The rotational movement of oscillating mass 12 around oscillation axis 24, e.g., the associated angular velocity, is thus ascertainable by analyzing capacitances C1 of associated capacitors 30.
If yaw-rate sensor 10 experiences a rotation around a sensitive axis 32 of yaw-rate sensor 10 during a rotational movement of oscillating mass 12 around oscillation axis 24, Coriolis forces Fc act on oscillating mass 12, which cause an additional rotational movement of oscillating mass 12 around a rotational axis 34. See, e.g., FIG. 1B. One may also refer to the rotational movement of oscillating mass 12 around rotational axis 34 as tilting/pivoting of oscillating mass 12 in relation to a surface of substrate 18.
Tilting/pivoting of oscillating mass 12 around rotational axis 34 causes a distance reduction of a first end 36 of oscillating mass 12, which is spaced apart from rotational axis 34, and a distance increase of a second end 38, which is diametrically opposite to first end 36 of oscillating mass 12 in relation to substrate 18. In order to ascertain the increase and decrease in the distances of ends 36 and 38 to substrate 18, counterelectrodes 40 are formed on substrate 18 to ends 36 and 38 of oscillating mass 12. Each of ends 36 and 38 is coupled to associated counterelectrode 40 on a power circuit having a capacitor 42. The increase or decrease in the distance between one end 36 or 38 and associated counterelectrode 40 thus causes a change in a capacitance C2 of associated capacitor 42. The change in capacitance C2 is proportional to Coriolis forces Fc. Correspondingly, the change in capacitance C2 is also a function of the yaw rate of the rotational movement of yaw-rate sensor 10 around sensitive axis 32. By analyzing capacitances C2 of capacitors 42, the yaw rate of the rotational movement of yaw-rate sensor 10 around sensitive axis 32 may thus be determined.
FIG. 2 shows a top view of an oscillating mass of a second typical yaw-rate sensor having two sensitive axes to explain its functionality.
The illustrated disc-shaped oscillating mass 52 of yaw-rate sensor 50 has drive and sensor electrodes 20 and 26. Drive and sensor electrodes 22 and 28, which work together, are fixedly connected to a substrate (not shown) of yaw-rate sensor 50 adjacent to drive and sensor electrodes 20 and 26 of oscillating mass 52. Because the functionality of drive electrodes 20 and 22 for setting oscillating mass 52 into a rotational movement around an oscillation axis (not shown) and the functionality of sensor electrodes 26 and 28 for ascertaining the angular velocity of the rotational movement around the oscillation axis.
Oscillating mass 52 is connected via four meandering springs 54 to a hub 56. With the exception of hub 56, no further components of the holding device of yaw-rate sensor 50 are shown in FIG. 2. Springs 54 are designed in such a way that oscillating mass 52, which rotates around the oscillation axis, is tiltable/pivotable in relation to the holding device via both rotational axes 58 and 60. A tilting movement of this type around at least one rotational axis 58 or 60 is executed by oscillating mass 52 if yaw-rate sensor 50 experiences a rotation around at least one of its two sensitive axes during the rotational movement of oscillating mass 52 around the oscillation axis. A rotation of yaw-rate sensor 50 around the first sensitive axis lying on first rotational axis 58 causes oscillating mass 52 to tilt/pivot around second rotational axis 60. Correspondingly, a rotation of yaw-rate sensor 50 around the second sensitive axis lying on second rotational axis 60 results in tilting of oscillating mass 52 around first rotational axis 58.
This tilting/pivoting of oscillating mass 52 around at least one rotational axis 58 or 60 may be established in the way already described above with the aid of counterelectrodes (not shown), which are situated on the substrate. Yaw-rate sensor 50 thus has the advantage that a rotational movement of yaw-rate sensor 50 around an axis which lies in the plane spanned by the two sensitive axes is detectable.
However, for the oscillating mass 52 to be pivotable around both rotational axes 58 and 60, springs 54 must have the lowest possible bending stiffness and a comparatively low torsional stiffness. In order to ensure the low bending stiffness and the advantageous torsional stiffness of springs 54, springs 54 having a comparatively long minimum length and/or a meandering shape are typically necessary. A long minimum length of linear springs increases the total extension of yaw-rate sensor 50 significantly and thus makes it more difficult to situate yaw-rate sensor 50 on a rotatable body.
If meandering springs are used, a precise design layout of the yaw-rate sensor is made significantly more difficult. A meandering spring 54 also has an asymmetrical mass distribution along its longitudinal axis. The asymmetrical mass distribution of the at least one spring 54 may result in false results of yaw-rate sensor 50, for example, ascertainment of rotational movements which have not been executed. This is also referred to as disadvantageous cross-sensitivity of yaw-rate sensor 50 or crosstalk of the measuring signals detected with the aid of yaw-rate sensor 50.
Furthermore, the disadvantageous spring stiffness and/or torsional stiffness, which frequently still exists in spite of a relatively great length and/or a meandering shape of spring 54, requires a comparatively high drive voltage of the electrostatic drive having drive electrodes 20 and 22. However, applying such a high drive voltage places special requirements on the electrostatic drive and thus prevents a cost-effective embodiment of the electronic drive circuit.
The problem described on the basis of yaw-rate sensor 50 frequently also occurs in a yaw-rate sensor having only one sensitive axis.